1. Field of the Invention
The present invention relates to a supply and exhaust pipe for an appliance with burning, or a supply and exhaust pipe for ventilation by taking fresh air into a room and/or emitting air from the room to the outside, the supply and exhaust pipe being used for constructing a duct by adding and connecting regular size pipes at site. More specifically, the present invention relates to a supply and exhaust pipe as mentioned having increased bending strength and sealing efficiency at joints when connecting supply and exhaust pipes, and a drop-off prevention function, particularly effective for pipes with a large diameter.
2. Description of the Related Art
For an appliance with a burner placed indoors, particularly for combustion exhaust, exhaust pipes (funnels) are usually constructed at the work site. In addition to such an exhaust pipe, there are some cases of installing a feed pipe or taking fresh air for combustion into an appliance with burner independently of an exhaust pipe. Alternatively, in some cases, a supply and exhaust pipe with a double structure, designed for exhausting through an inner pipe, while taking fresh air through the outer pipe, may be installed. Furthermore, in some cases, for indoor ventilation, in order to emit the air from the room to the outside and to intake fresh air into the room, supply and exhaust pipes may be constructed at the work site.
In these cases of such supply and exhaust pipes, typically, pipes produced at factories, such as straight pipes, elbow pipes, flexible pipes or the like, may be added and connected at site to construct a duct, the duct may have a number of joints created, since the structure of the joint is in a manner of inserting a pipe into an end of another pipe, the following aspects are required.
1. Bending Strength Shall be Large at Pipe Joints.
When extending a duct by connecting pipes, if external force is applied to the duct, stresses across the duct are concentrated to joints, so that these joints may have a buckling, a large bending, disconnection, or decreased sealing efficiency. In order to improve the stress, the insert amount is set, for example, around 75 mm, in a case of a pipe of SUS316L with a plate thickness of 0.3 mm and with a diameter of 120 mm, in addition, a pipe edge of an insert portion side is tapered inwardly to form a taper section, intending to improve buckling strength. The stress, in this case, indicates 0.2 kg to 0.3 kg as static loading, satisfying JIS safety standards in Japan.
However, when performing an impact test in accordance with the standards in the U.S.A. regarding exhaust duct (pipe), “UL1748 Venting System for Gas Burning Appliances, Categories II, II, and IV”, the results cannot satisfy these.
2. Sealing Efficiency Shall be High.
Particularly for pipes emitting combustion exhaust to the outside, emitted from a appliance with burner installed indoors, an increase in sealing efficiency at joints is an object. As well as under the normal conditions but additionally when the pipe itself may be deformed, when an external force is applied to the joints, and a bending occurs on the joints, sealing efficiency is decreased substantially. Consequently a leak of exhaust gas may occur.
The current sealing materials used to stop this trouble instantaneously are cross-sectionally circle shaped O-rings. When using this O-ring, as it is necessary to ensure a sufficient close contact between a pipe and an O-ring, the clearance of the insert portion to the diameter (width) of the O-ring is set small. For this reason, insertion of a pipe requires a large force. The O-ring is applied with a lubricant such as grease to facilitate its insertion. However, this lubricant is not only messy (it gets one's hands dirty), but also greasy and slippery, so that there is a need for frequent wiping out grease from hands and cleaning, etc. resulting in decreased work efficiency. To facilitate insertion of a pipe, another method may be considered to set the clearance not so narrow, however, this results in decreased sealing efficiency, particularly when a bending force is applied.
3. Pulling Strength Shall be High, Particularly on Pipes with a Large Diameter.
Joint portions of supply and exhaust pipes should be inserted easily and should not be easily withdrawn. Because of this, supply and exhaust pipes, particularly those emitting combustion exhaust, are not only just inserted, but also generally have a so-called drop-off prevention mechanism for giving a lock between pipes, to secure an inserted pipe not to be dropped off.
However, some drop-off prevention mechanisms have a structure that once it is locked, it cannot be unlocked later, which is inconvenient, accordingly, there are some provisions of drop-off prevention mechanisms having a configuration such that a locking system can be unlocked by one push.
Japanese Patent Publication No. 3 163008 relates to a supply and exhaust pipe of a regular size, comprising a receptacle for a joint formed on one end, and an outer bead formed on an insert portion formed on the other end. The pipe has a structure that an opening is provided on part of the external circumference of the receptacle, the external side of this opening has a stopper ring externally inserted pivotably over an area crossing the opening. Part of this stopper ring is punched out in “]” (bracket) shape as to face circumferentially, the edge of this “]” shape strip formed by punch is directed inwards to form a pawl. By pivoting the stopper ring from which this pawl is formed towards the edge of the pawl, the edge of the pawl is entered into the bead from the opening on the receptacle to prevent drop off. By pivoting the stopper ring reversely from the previous direction in turn to slide up the edge of the pawl with the edge of the opening, the edge of the pawl is disengaged from the outer bead to unlock the drop-off prevention.
However, in this known example, since the stopper ring has to be attached to the pipe, this lead to a high cost, and since the structure let the pawl of the stopper ring slide in-and-out to the outer bead, engagement force between the pawl and the outer bead is small, so that its pulling strength is small.
Japanese Patent Laid-open Publication No. 2000-161570 relates to a drop-off prevention system for supply and exhaust pipes having large pulling strength and allowing pipes detachable by one push. In this system an opening is formed on the receptacle side, and a lock member forming a lock section and an engagement is slidably provided. After the lock section is engaged to a circumferential trough at the opening by inserting the insert portion into the receptacle, by slightly pulling the insert portion side and by engaging the edge of the engagement of the lock member to the envelope wall, when pulling force is applied to the pipe, a wedge effect can work on the lock section so that the strength against pulling is improved. Furthermore, this configuration allows the connection of pipes to be disconnected simply by slightly pushing the insert portion side, pressing the stopper of the lock member, and pulling the pipe. However, the lock member of this invention has a structure to increase the lock force by a wedge effect. As such, most of the lock member is exposed to the outside of the pipe, and may be deformed by external forces In this way, if the lock member is deformed, operation by use of one's finger tip may become difficult and the lock cannot be disengaged.
Japanese Patent Laid-open Publication No. H10-26342 relates to a connecting system for supply and exhaust pipes having a simple structure with low cost in preventing drop-off and with some unlocking ease. It has a configuration that an outer bead on a receptacle is provided with an unlocking hole, and an insert portion is provided with an inner bead and a stopper pawl. When inserting the insert portion into the receptacle, the edge of the stopper pawl engages the outer bead to prevent drop-off. When withdrawing them, from the unlocking hole, an unlocking pin of the unlocking jig is pushed in, to disengage the stopper pawl. However, in the case of this drop-off prevention mechanism, to unlock the locking system, in each case the use of a separate jig is required. This is inconvenient.
In this connection, regarding the supply and exhaust pipes, the standards of Japan for products having a drop-off prevention mechanism are defined in JIS 52080 8.3 Pulling Strength (for products having a drop-off prevention mechanism), in which when a pulling force of 0.3 kN is applied to the disconnecting direction, the exhaust pipe shall not have deformation, damage, or disconnection at joints.
On the other had, the standards of the U.S.A. are defined in UL 1738 20.2 longitudinal force test, its contents are described in the following. Under the conditions described below, when a pulling force of 100 pounds or more (445 N) is applied, the product shall not be broken, removed, or damaged.
(1) Pulling force is to be applied to joints of the supply and exhaust pipes, and a test is carried out for each diameter of supply and exhaust pipes to be connected.
(2) Force is to be applied to the disconnecting direction.
(3) If the pipes are connected with adhesive or the like, the adhesive shall be dried for 24 hours prior to the test.
(4) Force is to be applied for 5 minutes.
Regarding the patent documents discussed above (Japanese Patent Publication No. 3 163008, Japanese Patent Laid-open Publication No. 2000-161570 and Japanese Patent Laid-open Publication No. H10-26342), as a common disadvantage, these can be used for pipes with a pipe diameter up to about 120 mm, however, in pipes with a larger bore, the engagement width of a pawl is relatively small to the circumference, so that pulling strength becomes smaller, leading to the pipe dropping off easily.